The invention relates generally to semiconductor fabrication and more specifically to in-line metrology for process control during wafer processing.
During semiconductor fabrication there are many opportunities for measuring features of substrates undergoing processing operations. Many of the features can be determined by capturing a signal indicating the feature. For example, various end point determination methods are available that employ laser interferometry, optical emission, etc. However, as the features continue to decrease in size, especially the thickness of films employed in the manufacture of semiconductors, the signals that are indicative of the feature become undetectable in certain situations. For example, eddy current sensors are used for displacement, proximity and film thickness measurements. The sensors rely on the induction of current in a sample by the fluctuating electromagnetic field of a test coil proximate to the object being measured. Fluctuating electromagnetic fields are created as a result of passing an alternating current through the coil. The fluctuating electromagnetic fields induce eddy currents which perturb the applied field and change the coils inductance.
FIG. 1 is a simplified schematic diagram of the principle upon which an eddy current sensor operates. An alternating current flows through coil 100 in close proximity to conducting object 102. The electromagnetic field of the coil induces eddy currents 104 in conducting object 102. The magnitude and the phase of the eddy currents in turn effect the loading on the coil. Thus, the impedance of the coil is impacted by the eddy currents. This impact is measured to sense the proximity of conducting object 102 as well as a thickness of the object. Distance 106 impacts the effect of eddy currents 104 on coil 100, therefore, if object 102 moves, the signal from the sensor monitoring the impact of eddy currents on coil 100 will also change.
Attempts to use eddy current sensors to measure thickness of a film has resulted in limited success. Since the signal from the eddy current sensor is sensitive to both the thickness of the film and distance of the substrate to the sensor, there are two unknowns that must be resolved. FIG. 2A is a schematic diagram of a wafer carrier having an eddy current sensor for measuring the thickness of a wafer during a chemical mechanical planarization process (CMP). Wafer carrier 108 includes eddy current sensor 110. During a CMP operation, wafer 114 supported by carrier film 112 of carrier 108 is pressed against pad 116 to planarize a surface of the wafer. Pad 116 is supported by stainless steel backing 118.
One shortcoming of the configuration of FIG. 2A comes from the variability of the carrier film, which, being only 0.020″ thick can undergo variations up to 0.006″ from sample to sample. Process conditions, in particular, film compression due to wafer load, affect the sensor-metal layer distance. Thus, the carrier film and variable process conditions cause a substantial variability in the distance between the wafer and the sensor. Accordingly, it becomes extremely difficult to calibrate for all the variables that effect the distance, which in turn impacts the thickness measurement of the sensor. Another shortcoming of this configuration is caused by the presence of another conducting material separate from the conducting material being measured and is commonly referred to as a third body effect. If the thickness of the conductive layer is less than the so-called skin depth, the electromagnetic field from the coil will not be completely absorbed and will partially pass through to stainless steel backing 118 of pad 116 of FIG. 2A. The electromagnetic field will induce additional eddy currents within the stainless steel belt, thereby contributing to the total signal from the eddy current sensor.
Furthermore, it should be appreciated that the pad wears or erodes over time, causing variation in the distance between the stainless steel backing and the eddy current sensor, which influences the appropriated contribution to the total eddy current sensor signal. Thus, a wear factor has to be considered as the wafers are continuously being processed. Consequently, due to the variability injected into the thickness measurement, the amount of error is unacceptably high and unpredictable. Furthermore, the focus on uniformity of removal rates is misguided for current applications. That is, from the end user's perspective, it is desired to have a uniform end layer on the surface of the semiconductor substrate which is not necessarily the result from a uniform removal rate. For example, if the surface of the processed wafer prior to planarization is not uniformly thick, the non-uniformities are maintained when a uniform removal rate is applied to the processed wafer.
FIG. 2B is a simplified cross sectional schematic diagram illustrating the results of applying a uniform removal rate to a surface of a silicon substrate. As can be seen, a uniform removal rate applied to substrate 109 results in substrate 11 having a non uniform thickness, i.e., the thickness at the center of substrate II is smaller than the thickness at the outer edge of the substrate, which is similar to substrate 109 prior to the planarization application. As a result, the center of substrate 111 may be over-polished especially with respect to a copper clearing process.
Additionally, some systems that are sensitive enough to accommodate the low film thickness required for semiconductor processing may have their sensing signals interfered with from third bodies. For example, the signal may be impacted by the presence of other conductive objects within the sensing vicinity. Furthermore, the impact may not be constant, but dependent on the metal film thickness, thereby making direct signal-to-thickness conversion impossible.
In view of the foregoing, there is a need to provide a method and apparatus that may deliver a uniform thickness rather than a uniform removal rate to provide control over the uniformity of the targeted remaining layer thickness for the wafer. In addition, there is a need to provide real time differential closed loop control for the remaining thickness of a semiconductor wafer being processed.